1. Field of the Invention
The present invention relates to the preparation of compounds containing isocyanurate groups by the catalytic cyclotrimerization of monoisocyanates or polyisocyanates, and, more especially, to such process wherein the catalysis is initiated by contacting the isocyanate with an aminosilyl compound, such as an aminosilane, a silylurea compound or a silazane compound.
2. Description of the Prior Art
Numerous processes are known to this art for the preparation of simple or polymeric compounds containing isocyanurate groups by the catalytic cyclotrimerization of aliphatic, cycloaliphatic or aromatic isocyanates. The cyclotrimerization reaction can be either complete or partial. Thus, monoisocyanurates can be obtained from monoisocyanates, while polyisocyanate/polyisocyanurates will be prepared from polyisocyanates by partial cyclotrimerization, and polyisocyanurates devoid of free isocyanate groups will be prepared from polyisocyanates by total, or complete cyclotrimerization. Polyisocyanurates without free isocyanate groups constitute a class of polymeric materials very frequently used in cellular form (polyisocyanurate foams).
As regards the preparation of isocyanurate or polyisocyanurate by the total trimerization of aliphatic, cycloaliphatic or aromatic monoisocyanates or polyisocyanates, the following compounds are exemplary catalysts therefor: tertiary amines, phosphines, alcoholates, and alkali metal or alkaline earth metal derivatives, such as their oxides, their hydroxides, their carboxylates, and the like. These catalysts are described, for example, in Journal of Cellular Plastics, pp. 85-90 (January, 1965), and Journal Macromolecular Science Reviews; Macromolecular Chemistry (5/1), 105-109 (1970). Certain silicon and tin derivatives have also been described as catalysts for the cyclotrimerization of phenyl isocyanate to provide isocyanurate. Thus, Itoh, Matsuzaki and Ishii demonstrated the catalytic role of bis-trimethylsilyl sulfide [Journal Chemical Society (C), pp. 2,709-2,712 (1981)] and N-silylamines [Journal Chemical Society (C), pp. 2,005-2,007 (1969)]. It will be noted that the trimethylsilylamines make it possible to cyclotrimerize isocyanatobenzene very slowly to yield isocyanurate (experiments carried out for 42 hours at 150.degree. C.). Silylamines, therefore, appear to be cyclotrimerization catalysts displaying very little activity for aromatic isocyanates. Furthermore, the above-mentioned authors also demonstrated that disilazanes (hexamethyldisilazane, heptamethyldisilazane and hexamethyl-N-ethyldisilazane) were not catalysts for the cyclotrimerization of phenyl isocyanate to yield triphenyl isocyanurate: to the contrary, these silazanes proved to be reagents which led, with phenyl isocyanate, to iminotriphenylhexahydrotriazine-diones [for example, 4-methylimino-1,3,5-triphenyl-hexahydro-1,3,4-triazine-2,6-dione and 1,3,5-triphenyl-2,4,6-tris-(phenylimino)hexahydro-1,3,5-triazine are obtained from heptamethyldisilazane].
Polyisocyanate/polyisocyanurates, which are base constituents for varnishes and paints, are typically obtained by the partial cyclotrimerization of the NCO groups of simple aliphatic or aromatic polyisocyanates, or of polyisocyanate adducts, with the aid of a variety of catalysts, such as tertiary amines [German Pat. No. 951,168], alkali metal or alkaline earth metal derivatives, such as hydroxides, carbonates and alcoholates [French Pat. No. 1,190,065], quaternary ammonium hydroxides [French Pat. Nos. 1,204,697 and 1,566,256 and published European Patent Application Nos. 00/03,765 and 10,589], phosphines [French Pat. Nos. 1,510,342 and 2,023,423 and published German Patent Application No. 1,934,763], catalysts containing an ethyleneimine group [French Pat. Nos. 1,401,513 and 2,230,642] and finally Mannich bases [French Pat. Nos. 2,290,459 and 2,332,274]. If appropriate, these various catalysts can also be associated with a carbamic acid ester [tertiary amine+carbamate: French Pat. No. 1,172,576; alkali metal or alkaline earth metal derivatives+carbamate: French Pat. No. 1,304,301; or Mannich base+carbamate: French Pat. No. 2,290,459].
The catalyst for the partial cyclotrimerization of the NCO groups must typically be deactivated when the desired proportion of free isocyanate groups has been reached. This deactivation can be effected by adding to the reaction mass an acid compound (a hydracid, an acid chloride or the like), an alkylating agent (methyl iodide or the like) or an acylating agent. Finally, the deactivation can be carried out by means of a suitable heat treatment.
Thus, various catalyst systems were known which made it possible to trimerize aliphatic or cycloaliphatic isocyanates to provide isocyanurate compounds (simple isocyanurates or polyisocyanate/polyisocyanurates), but these various systems are all characterized by one or more serious disadvantages. Catalysis by alkali metal and alkaline earth metal derivatives, for example, always takes place unpredictably, with a certain time delay, and is then sudden and generally difficult to control because of its excessively high activity. With less reactive catalyst systems, the delay in the catalytic activity is no longer observed, but the catalysis is relatively ineffective. It is necessary to cyclotrimerize at a high temperature, and this causes the formation of a considerable amount of dimer. Furthermore, it is generally necessary to heat during the period of deactivation of the catalyst, and this also favors the formation of dimer.
Serious need therefore exists in the art for a catalyst system which make it possible to cyclotrimerize aliphatic or cycloaliphatic isocyanates to give compounds containing isocyanurate groups, the catalytic reaction taking place without an induction period and in a uniform manner at a relatively moderate temperature. It is furthermore important for any catalyst of such desired type to be readily deactivated.